1. Field of the Invention
The present invention relates to an optical disk and a method of manufacturing the same, and, more particularly, to an optical disk which can ensure high-density recording and a method of pre-formatting this optical disk.
2. Description of the Related Art
As a vast amount of information is handled these days, people pay greater attention to optical disks as recording media for saving such information. Because each optical disk has record marks of nearly the same size formed on the recording film by a laser beam which is focused to a spot of about 1 .mu.m in diameter, very high density recording is possible. Optical disks are classified to a read only type typified by a compact disk or a laser disk, a write once type on which information can be written only once as popular in an electronic filing system or the like, and a rewritable type on which information can be rewritten as one used in a hard disk drive or a floppy disk drive. There are two recording systems for the rewritable optical disk: the magneto-optic recording system which applies a magnetic field to the vertical magnetic film and irradiates a laser beam thereon to record or erase information and the phase change recording system which irradiates a laser beam on the recording film to selectively change the state of the recording film between amorphous and crystalline, thereby recording or erasing information.
Due to the recent improvement on the recording density of those optical disks, the amount of embossed pit information previously written or pre-formatted on the optical disks is increasing more and more. FIGS. 1A and 1B show one example of a train of pits. As shown in FIGS. 1A and 1B, a preamble portion formed by the pre-formatting has a densest pattern where pits P1 of the unit pit length are arranged at intervals equal to the pit length and a thinnest pattern, following the densest pattern, where pits P2 whose length is an integer multiple of the unit pit length are arranged at intervals equal to the unit pit length. With such a pit pattern, when the pit intervals of the densest pattern are wide enough for the light beam spot, the light beam spot B.sub.S is not generally irradiated on two pits P1 and P2 at the same time as shown in FIG. 1B. Therefore, the amplitude of a reproduced signal from the densest pattern and the amplitude of a reproduced signal from the thinnest pattern do not change much as shown in FIG. 1C, and the slice level Th for digitization can be approximately constant as indicated by the broken line in FIG. 1C.
As opposed to this pattern, when the pit length of the pits P1 in the densest pattern is made narrower than the size of the beam spot and the intervals of the pits P1 are narrowed to be equal to the shortened pit length as shown in FIGS. 1D and 1E, the light beam spot a1 is simultaneously irradiated on two adjoining pits as indicated by the solid line in FIG. 1E. As shown in FIG. 1F, therefore, the top level of the reproduced signal from the densest pattern becomes lower and the bottom level becomes higher. Consequently, the amplitude of the reproduced signal from the densest pattern becomes smaller than that of the reproduced signal from the thinnest pattern.
When the pit intervals of the densest pattern are wide enough for the light beam spot as shown in FIG. 1C, the amplitudes of the reproduced signal obtained by continuous reproduction from the densest pattern to the thinnest pattern become approximately the same. In setting the slice level for digitizing the reproduced signal, therefore, the slice level Th does not change much at the transitional point from the densest pattern to the thinnest pattern and is almost constant as already mentioned. When, as opposed to the case of FIG. 1C, the pit length and the pit intervals of the pits in the densest pattern are set much smaller than the size of the beam spot, the amplitude of the reproduced signal from the densest pattern differs significantly from the amplitude of the reproduced signal from the subsequent thinnest pattern. Therefore, the slice levels Th1 and Th2 always set approximately at the centers of the amplitudes vary considerably, the signal reproduction cannot follow up the change in slice level at the transitional point of the pit pattern so that a train of pits in the thinnest pattern cannot be reproduced accurately and stably.